The invention relates generally to transfer cases for four-wheel drive vehicles and more specifically to a transfer case having a center differential which is configured to selectively provide a four-wheel underdrive operating mode.
The obvious benefits of improved traction and vehicle control achieved by four-wheel drive systems in adverse driving conditions such as snow, freezing rain, ice and even water has been known and appreciated by vehicle designers for many years. A not so obvious benefit relates to the use of four-wheel drive when towing a trailer carrying a boat, snowmobiles and the like. Here, too, the improved traction provides improved driver control and stability in adverse driving conditions. When trailer towing is viewed as a specific operational mode, additional features and requirements may be added to the list of vehicle design criteria. For example, when the vehicle weight is augmented by several hundred or several thousand pounds, a slightly higher overall drive ratio provides improved torque and thus improved acceleration.
In contrast, when the vehicle is utilized in two-wheel drive the presumption exists that neither the enhanced traction nor improved acceleration would be beneficial or necessary inasmuch as either the vehicle is not towing a trailer, the road conditions are good or both.
The present invention is directed a transfer case having a center differential which provides direct two-wheel drive and an underdrive four-wheel drive operating mode which improves vehicle performance.